1. Field of the Invention
The present invention relates to a pneumatic tire in which a white rubber is arranged in a side wall portion.
2. Description of the Related Art
Conventionally, in a tire which is mainly used in a high load vehicle such as a recreational vehicle (RV) or a light truck (LT) or the like, there has been known a pneumatic radial tire in which various white patterns are formed by a white rubber which is arranged in a side wall portion, and a fashion characteristic is enhanced (for example, refer to Japanese Unexamined Patent Publication Nos. 9-111041 and 2008-1742). The white rubber is generally arranged in a region which is closer to a bead portion than a tire maximum width position at which a deflection at a time of traveling is great.
The white rubber cannot employ a carbon black having a high reinforcing characteristic for holding the white color, and is accordingly softer and lower in rigidity than a normal rubber constructing the bead portion and the side wall portion. Therefore, a strain generated near the bead portion due to repeated compression and tension at a time of traveling tends to be concentrated on an interface between the white rubber and a rubber jointed thereto, and causes a separation of a roll-up ply, and there is a risk of running into a malfunction.
As a method of absorbing the strain mentioned above, there can be considered to stick a rubber reinforcing tape to the roll-up ply. However, according to this method, since there is a tendency that a member thickness corresponding to the stuck reinforcing tape is increased and a heat generation characteristic rises up, there is a problem that a durability is lowered under a high-speed traveling condition in which an amount of heat generation near the bead portion rises.
In Japanese Unexamined Patent Publication Nos. 5-16618, 8-175129, and 10-29413, there is described a pneumatic tire in which a bead filler is divided into two or three sections in a diametrical direction, and a rubber in an outer peripheral side is made softer than a rubber in an inner peripheral side. However, since these tires accompany a low rigidity of the side wall portion caused by a significant reduction in hardness of the bead filler, it is not possible to sufficiently suppress a heat generation rise caused by a standing wave in a high speed region, and there is a risk that a deterioration of high-speed durability is caused. Further, there is a risk that a handling performance is lowered by the low rigidity of the side wall portion. As a matter of course, these structures are not provided on the assumption of the structure in which the white rubber is arranged in the side wall portion.